QUATERNARY STRATIGRAPHY AND NEOTECTONICS OF RAPIDES PARISH, LOUISIANA: A CRITICAL REEXAMINATION OF HAROLD N. FISK'S TERRACE STRATIGRAPHY AND NOMENCLATURE

In his monograph “Geology of Avoyelles and Rapides Parishes” (1940), Dr. Harold N. Fisk subdivided the Quaternary sediments in Rapides Parish into his classic Williana, Bentley, Montgomery, and Prairie formations. Fisk mapped these formations primarily on the basis of what he interpreted to be terrace surfaces and escarpments that separated these surfaces using available planimetric and 15-minute topographic maps and, in the field, barometric altimeters. From the mapping of these terrace surfaces, Fisk concluded that significant Quaternary folding had occurred in Rapides Parish.

As part of STATEMAP funded mapping by the Louisiana Geological Survey for the Alexandria 30 x 60 degree geologic quadrangle, the author analyzed surface morphology using LIght Ranging And Detection (LIDAR) Digital Elevation Models (DEMs) and subsurface stratigraphy using well logs and foundation borings. The Williana and Bentley type areas as designated by Fisk lie partly in the extent of this quadrangle. Analyses of the LIDAR DEMs found that areas mapped by Fisk as the Williana and Bentley formations in Rapides Parish are characterized by deeply eroded ridge and ravine topography lacking discernible terrace surfaces and associated escarpments. There is a lack of any subsurface evidence for the presence of the separate terrace deposits and inferred bounding unconformities that Fisk used to define these formations. Thus, the primary criteria that Fisk used to differentiate his Williana Formation from his Bentley Formation are nonexistent. As a result, the Williana Formation has been abandoned by this study as a valid stratigraphic unit. North of the Red River in this region, sediments that Fisk mapped as his Williana and Bentley formations are reclassified as the Bentley Alloformation. South of the Red River in Rapides Parish, sediments that Fisk mapped as his Williana and Bentley formations are reclassified as the Lissie Alloformation. Within Rapides Parish, terraces underlain by sediments of either the Montgomery alloformation or Prairie Allogroup lie along the sides of the Red River alluvial valley and its tributaries. Fault-line scarps that occur in southern Rapides Parish are the only evidence of Quaternary tectonic activity in it.